Solid laser with an emission wavelength of 0.5-0.65 micrometers

ABSTRACT

The invention relates to a solid laser with an emission wavelength lying between 0.5 and 0.65 μm. A chromium-doped Mg 2  SiO 4  (forsterite) laser rod is pumped by a laser diode emitting between 0.75 and 0.8 μm, this laser diode having an active layer which is Ga 1-x  Al x  As based, with x lying between 0.1 and 0.18. The laser device finds particular application in the isotopic separation of uranium.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to a solid laser with an emission wavelength lyingbetween 0.5 and 0.65 micrometers.

Such a laser is particularly suitable for use in isotopic separationprocesses.

2. Discussion of the Background

Processes for separating uranium isotopes by laser have been studied forseveral years. Their implementation requires the selective excitation ofthe uranium isotopes from laser sources, the frequency of which must beparticularly well adjusted and controlled.

Among the methods which have been studied hitherto, there are thosewhich use sources emitting at around 16 μm and those which emit between0.55 μm and 0.65 μm. This latter approach uses a copper vapour type oflaser to pump a dye laser, the technology of which is quite critical.

The renewal of interest in solid lasers, particularly because of thepossibilities offered by pumping with laser diodes (efficiency,compactness, lifetime, reliability, etc.) opens the way to new methodsfor separating uranium isotopes. Certain methods based on solid lasershave already been proposed.

However, these methods lead to bulky and expensive devices.

SUMMARY OF THE INVENTION

The invention therefore relates to a laser emitting at a wavelengthlying between 0.55 μm and 0.65 μm and overcoming these disadvantages.

The invention therefore relates to a solid laser with an emissionwavelength of 0.5-0.65 micrometers, characterised in that it comprises:

a laser rod based on chromium-doped Mg₂ SiO₄ placed in a resonantcavity;

at least one laser diode emitting towards the rod a pumping beam ofwavelength lying between 0.75 and 0.8 micrometers;

a frequency doubler crystal receiving a beam emitted by the laser rodand emitting in exchange a beam of wavelength 0.5-0.65 micrometers.

BRIEF DESCRIPTION OF THE DRAWINGS

The various objectives and features of the invention will appear moreclearly in the description which will follow, given by way of example inwhich reference is made to the appended figures, in which:

FIG. 1 represents a first example of an embodiment of the deviceaccording to the invention;

FIG. 2 represents a variant of an embodiment of the device of FIG. 1;

FIG. 3 represents a second example of an embodiment of the deviceaccording to the invention;

FIG. 4 represents another variant of an embodiment of the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

According to the invention, provision is made for placing a rod ofchromium-doped forsterite (Mg₂ SiO₄) in an optical cavity and forexciting it using a laser diode emitting at 0.8 μm. This type of laserdiode was designed especially for the present invention. It is producedfrom a ternary compound Ga_(1-x) Al_(x) As and its composition is suchthat

    0.1<×<0.18.

This material is the location of the laser emission by recombination ofelectron-hole pairs. It is built into a structure of the opticalwaveguide type by inserting an active layer of Ga_(1-x) Al_(x) Asbetween two layers of Ga_(1-y) Al_(y) As of aluminium composition suchthat 0.2<y<0.4.

The wave generated by the laser rod 1 excited by the laser diode has awavelength covering the spectrum lying between 1.1 and 1.3 μm.

This wave emerging from the optical cavity is then doubled in frequencyin a non-linear crystal.

The chromium-doped forsterite (Cr: Mg₂ SiO₄) laser has an emissionspectrum at room temperature ranging from 1.167 μm to 1.345 μm with apeak in the emission centred on 1.221 μm. Its absorption spectrum rangesfrom 0.4 to 1.1 μm with a maximum located at around 0.75 μm. Pumping itby a laser diode emitting at the wavelength of 0.8 μm is thereforeentirely appropriate.

In addition, a direct pumping by laser diodes operating at about 0.8 μmenables a high-efficiency "solid state" source to be produced.

In fact, the lifetime of the fluorescence is of the order of 15 μs,which is compatible with laser diodes. It is particularly suitable forpumping by laser diodes, especially when operation at a high pulse rateis desired. In this case, the replenishment rate (duration ofexcitation, repetition frequency) is an important parameter that shouldbe taken into account.

Such a frequency-doubled source enables the emission wavelength to bematched to that which must be used in operations involving the selectiveexcitation of uranium isotopes in the 0.55-0.65 μm band.

Several crystals can be envisaged in this doubling operation, such as,for example:

LiNbO₃ ;

KTiOPO₄, known under the name KTP;

l-arginine phosphate and its deuterated compound D - LAP known under thename LAP.

This crystal therefore has, potentially, the great advantage of beingtunable over the spectral window of the near infrared which, doubled infrequency, covers the spectral range which must be explored for theapplication mentioned hereinbefore.

FIG. 1 represents a first example of an embodiment of the laser deviceaccording to the invention.

A rod 1 of chromium-doped forsterite (Mg₂ SiO₄) is placed in an opticalcavity consisting of one of the two mirrors 3 and 4. The optical cavitymay also be produced by two opposites faces of the rod.

The laser diodes 2.0, 2.1 to 2.n illuminate the rod 1 by means of beamswith wavelength 0.8 μm. These laser diodes are supplied with current bya modulator 7.

The light beam generated by the rod 1 and emerging from the laser cavity3, 4 is transmitted to a frequency doubler 5 made, for example, ofLiNbO₃ or BBO. (B_(a) B₂ O₄) The laser diodes are formed in thefollowing way:

The active part consists of a GaAlAs-based alloy with an aluminiumconcentration which makes it possible to have a laser emission for thepumping at . . . either at 0.8 μm (Ga.sub..9 Al.sub..1 As) or at 0.75 μm(Ga.sub..82 Al.sub..18 As).

According to the invention, the combination of a laser comprising thechromium-doped forsterite rod 1, the GaAlAs laser diodes 2.0 to 2.n andthe frequency doubler 5 makes it possible to obtain a light beam ofwavelength lying between 1.167 μm and 1.345 μm which, doubled by thefrequency doubler 5, gives a beam with a wavelength of about 0.6 μm(lying between 0.5 and 0.65 μm).

FIG. 2 represents a variant of the embodiment of the device of FIG. 1.According to this variant, the optical cavity 3, 4 incorporates atriggering device 6 (Q switch in English-language terminology).

FIG. 3 represents an example of an embodiment incorporating a laserdiode 2 illuminating the laser rod 1 through one of the ends of thecavity.

The laser diode 2 has the same constitution as the laser diodes 2.0, 2.1to 2.n) of FIGS. 1 and 2 and emits an excitation light beam ofwavelength 0.8 μm towards the forsterite laser rod 1. The device 6 is aQ-switch.

The beam emerging from the cavity 3, 4 containing the laser rod 1 andthe Q-switch 6 is transmitted to a frequency doubler 5 which supplies abeam at twice the frequency (wavelength 0.6 μm).

Just as with the laser diodes of FIG. 1, the laser diode 2 could becontrolled by a modulator. The optical cavity 3, 4 could then be formedby two opposite cleaved faces of the laser rod 1.

In the above, provision was made for the frequency doubler crystal 5 tobe located outside the cavity but, as is shown, it may also be placedwithin the laser cavity, preferably between the laser rod 1 and theQ-switching cell 6. According to this variant, the laser diodes may alsobe on both sides of the laser rod 1 or may surround the laser rod.

It is to be clearly understood that the above description has only beengiven as a non-limiting example and that other variants may be envisagedwithout going outside the scope of the invention. The numerical examplesand the nature of the materials indicated have only been provided inorder to illustrate the description.

We claim:
 1. A solid laser device with an emission wavelength between0.5 and 0.65 μm, comprising:a resonant cavity; a laser rod based onchromium-doped Mg₂ SiO₄ placed in said resonant cavity for emitting alaser beam; at least one laser diode emitting a pumping beam of awavelength between about 0.75 and 0.8 μm toward said laser rod; and afrequency doubler crystal receiving said laser beam emitted by saidlaser rod and emitting a beam of a wavelength between 0.5 and 0.65 μm;wherein an active layer of said at least one laser diode has acomposition of Ga_(1-x) Al_(x) As, where x is between 0.1 and 0.18, andsaid active layer is enclosed between two confining layers having acomposition of Ga_(1-y) Al_(y) As, where y is between 0.2 and 0.4. 2.The solid laser device according to claim 1, wherein said frequencydoubler crystal is BBO-based.
 3. The solid laser device according toclaim 1, wherein said frequency doubler crystal is LiNbO₃ -based.
 4. Thesolid laser device according to claim 1, further comprising a Q-switchlocated in said resonant cavity on the path of said laser beam emittedby said laser rod.
 5. The solid laser device according to claim 1,wherein said at least one laser diode is controlled by a control currentsupplied by a control current modulator.
 6. The solid laser deviceaccording to claim 4, wherein said frequency doubler crystal is locatedin said resonant cavity.